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How do we use the CMB to learn about the Universe? Category: Astronomy • Dark Energy • Dark Matter • Physics • big bang
Posted on: June 28, 2010 8:16 PM, by Ethan Siegel
The task is not so much to see what no one yet has seen, but to think what no body yet has thought about that which everyone sees. -Arthur Schopenhauer
Most of you who've been reading Starts With A Bang for a while have seen this picture come up many, many times.
Why do I keep putting it up, and why is it so important?
Let's go back to the 1960s for a little bit. Back then, there were two major rival theories about the origin of the Universe: the Big Bang theory and the Steady-State model of the Universe. The Big Bang contended that the Universe was hotter and denser in the past, and thus of a finite age, while the Steady-State theory contended that the Universe has always been the same temperature and density as it is today.
One huge difference between these theories? The Big Bang says that at one point in the Universe's past, it was so hot and dense that a thermal bath of radiation dominated everything, and that radiation -- untouched since the Universe cooled enough to form neutral atoms -- should still be leftover today.
It should be much colder now, only a few degrees above absolute zero, but it should still be around.
By contrast, the Steady-State model said that the only light that should be around is starlight, either coming directly from stars or reflected off of the gas throughout the Universe.
Well, in the 1960s, Arno Penzias and Bob Wilson used this piece of equipment (above) -- the Horn Antenna -- to measure microwave-frequency radiation. What they found was that radiation was coming from everywhere in the sky (that they could measure) with the same exact energy!
Over the past 45 years or so, we've confirmed that this energy is peaked practically exactly at 2.725 Kelvins -- just a slight bit above absolute zero -- everywhere in the sky! It corresponds identically to the leftover glow predicted by the Big Bang, and contradicts the predictions of Steady-State theory.
(The hardcore among you will see the 400-sigma error bars and gasp. That is not a typo!)
It wasn't until 2003 that a detailed map of the variations in the sky -- the departures from 2.725 Kelvin -- were measured. These fluctuations were only a few microKelvins, or millionths of a degree, and here's what they look like.
(This map, by the way, is the entire sky spread out into an ellipse, known as a Mollweide projection.)
These fluctuations will tell us a whole bunch of information about the Universe, including what types of structures will form, how old the Universe is, how much normal matter, dark matter, and dark energy are in it, how important neutrinos are, and many other things.
But not like this, they won't. You see, this map has an awful problem with it. Not only is our galaxy in the way,
But a bunch of strong extragalactic microwave- and radio-sources are out there too, mucking up our picture! So what do we need to do? We make a "mask" as best as we can, to remove the offending parts of the picture. (Shown in red, below.)
Without this data from the cosmic microwave background, we can still learn a whole lot about the Universe, including its age, the presence of dark matter and dark energy. But we couldn't do it as accurately as we can from the fluctuations in this microwave background.
Well, a new paper came out earlier this year, in which the authors claim to have found a problem with the mask used by looking at distant, luminous red galaxies.
My take on it? This is a real issue that needs to be understood. If we're using the wrong mask, that means we're either throwing out real data, which is bad, or we're keeping bad data, which is worse. The problem areas are, for example, circled in white in the image below.
And what this goes to show (by my accounts) is that we're not done fully analyzing this data yet! There's a lot to learn about the Universe, and this is the best data set we have (so far) to do so. That's why we'd better make sure we're getting it right!
And I don't have a problem, at all, with the way they present their results. And yet, I want to smack somebody because of this. Robert Massey, I am calling you out. From the PhysOrg website: Sawangwit and Shanks used astronomical objects that appear as unresolved points in radio telescopes to test the way the WMAP telescope smoothes out its maps. They find that the smoothing is much larger than previously believed, suggesting that its measurement of the size of the CMBR ripples is not as accurate as was thought. If true this could mean that the ripples are significantly smaller, which could imply that dark matter and dark energy are not present after all.
Well, let's see. The first sentence is good. The second sentence is also good. And the third sentence, which I've bolded, is a howling lie.
Why do this? Why ruin some good science by reporting an impossible conclusion? You could, at this point in cosmology, tell me that everything from the Cosmic Microwave Background is unreliable and meaningless, and we would still have overwhelming evidence for dark matter and dark energy. Why?
Because the prime evidence for dark matter doesn't come from the cosmic microwave background. It comes from galaxies, galaxy clusters and large-scale structure!
And the prime evidence for dark energy doesn't come from the cosmic microwave background, either. It comes from observations of supernovae and other very distant sources.
While the microwave background supports both of these, it isn't the foundation of a house of cards. Rather, it's an independent test that, thus far, has reached the same conclusions. And, having worked with this data myself, I can personally say I find it (and the various masks used) very compelling, and of overwhelmingly high quality.
So we've got to make sure we get the details ironed out so that we can have confidence in our confident claims, but we are way past the point where one bad data set will cause us to revise our picture of the Universe; the evidence is far too comprehensive and compelling at this point.
But we keep searching for anomalies and examining the suspicious ones we do find, because if we don't, that's when we stop learning. Just don't discount so easily what we already know!
http://scienceblogs.com/startswithabang/2010/06/how_do_we_use_the_cmb_to_learn.php |
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